THE STRUCTURE OF PLANTS. 



.9 



found in the root as well as in the stalk ; they 

 partly compose the nerves and veins of the leaves 

 and vessels of the corolla, and are found in the 

 stamens and pistils in the fruit, and also in the 

 umbilicus of the seed. These spirals, at their 

 extremities, terminate in the cellular tissue, ac- 

 cording to Mirbel ; but according to Dutrochet, 

 they end in a sort of cone, which is more or less 

 acute. 



If the root of the common garden lettuce is 

 cut partly across, and the remainder broken 

 gently asund*, the spiral vessels will most gen- 

 erally be discernible. They are not always 

 simple, but are sometimes found with double, 

 triple, or even with a great number of parallel 

 spirals. 



They may be also found in the leaf stalk of 

 the common artichoke, when young and fresh, 

 in the fibres of which, they are not only re- 

 markably large and distinct, but also remarkably 

 beautiful, some of them exhibiting in their na-- 

 tural position the appearance of spiral coats, 

 investing interior fibres, rather than that of fonn- 

 ing a distinct tube, and seeming, when uncoiled, 

 to be themselves formed of a sort of net work 

 membrane,consisting of three principal and longi- 

 tudinal fibres. They are discernible also in the 

 leaf as well as leaf stalk, though not quite so 

 easily detected. If a leaf is taken and gently 

 torn asunder in a transverse direction, there will 

 be seen fragments of the spiral tubes projecting 

 from the torn edges, and generally accompanying 

 the nerves. In the calyx and corolla of the 

 flower they do not exist so generally as in the 

 leaf, on which account, some botanists have de- 

 cided too hastily with regard to their non-ex- 

 istence in these parts. The calyic of the scahiosa, 

 and the corolla of the honeysuckle, will afford 

 examples. In whatever part of the plant they 

 are found to exist, they are always endowed with 

 a considerable degree of elasticity. For though 

 they be forcibly extended so as to undo the 

 spires, they will again contract and resume their 

 former figure, when the extending cause is with- 

 drawn ; and if they are even stretched till they 

 break, the fragments will again coil themselves 

 up as before. Hedwig considered the spiral 

 vessels composed of two parts : a straight and 

 central tube full of air, and of a tube rolled 

 spirally on the former, and full of aqueous fluid. 

 Others have considered them as formed of a 

 very thin external tube, in which a small silvery 

 layer is rolled spirally, in such a manner as to 

 keep its parietes or walls asunder ; while again, 

 some suppose that the spires of the vessel are 

 held together by a very thin membrane, which 

 is easily torn when the spiral thi-ead is unrolled. 

 From this it would follow, that the spirals 

 form continuous tubes. 



According to DecandoUe, the interior canal of 

 the spiral vessel, in its natural state, is always 



found free from water. It is true, that if a piece 

 of wood is dipped in water, this fluid penetrates 

 into the canal ; and when we permit coloured 

 fluids to flow into the cut branches of plants, 

 these fluids become apparent in the sides of the 

 spiral canals ; but they are also seen still more 

 distinctly, in the neighbouring bundles of sap 

 vessels, and they penetrate in considerable quan- 

 tity even into the cellular texture. We are not 

 therefore, entitled from this entrance of coloured 

 fluids, to conclude respecting the natural con- 

 tents of those canals, because in general this 

 penetration of coloured sap does not succeed in 

 an uninjured root. In sph-al canals which grow 

 rapidly, the fibres are often torn in such a man- 

 ner, that they fall together in the shape of rings. 

 These ring-shaped vessels, as they have been 

 called, are therefore an entirely accidental variety 

 of the primitive form of the spiral vessels ; and 

 this is the more evident, because we find the 

 same vessel in one situation as a spiral canal, 

 and in another as a ring-shaped vessel. This 

 change, besides, shows incontestibly that the 

 spiral vessels cannot conduct sap, since they are 

 often nothing else but rings at a distance from 

 one another. As then the spiral vessels and all 

 their varieties are uniformly found empty of 

 fluids, as they show themselves only in the higher 

 plants, and constantly appear wherever a strong 

 shoot is cut off ; as they are always in the com- 

 pany of the sap vessels, and as they maintain, 

 by their constant diagonal direction, the middle 

 situation between the perpendicular and hori- 

 zontal ; we must from all these considerations 

 conclude that they are the instruments of the 

 higher vital activity of plants, and that they 

 are the organs by which the sap tubes receive 

 an internal excitement to the speedy propulsion 

 of the sap. 



Mixed vessels, fig. «, are those which 

 are composed of two or more of the 

 foregoing varieties. Mirbel exempli- 

 fies this combination in the common 

 flowering rush, in which the porous, 

 spiral, and false spiral tubes appear 

 united into one. He seems, however, 

 to be of opinion, that the appearance 

 is to be regarded as being merely an 

 indication of the commencement of 

 e the process of union, of the contiguous 

 rings of the spiral tubes, by which they are to be 

 converted into a new form. Amici thinks that 

 the false spirals never become true ones ; and 

 he besides remarks, that these two kinds of 

 vessels occupy different places. * 



These various kinds of vessels thus united in 

 considerable numbers, form bundles connected 

 by cellular tissue ; they then form fibres pro- 

 perly so called ; and these fibres, or bundles of 

 tubes, constitute the frame work, and, as it 

 were, the skeleton of roost of the organs of 



